In this report information from the Health Examination Survey is used to estimate current age at menarche (onset of menstruation) of women in the United States and to examine the data for differences in age at menarche among subsets of the population and for changes in age at menarche over time. As a major program of the National Center for Health Statistics the Health Examination Survey collects and analyzes information gathered by direct physical examinations, tests, and measurements performed on probability samples of the U.S. population. "Supplementary health and social information is also collected by means of several questionnaires. The survey is conducted as a series of separate, cross-sectional programs referred to as "cycles." Each cycle is limited to some specific segment of the U.S. population and to certain aspects of the health of that segment. During the period 1960-1970 three separate cycles have heen completed. Information from all three cycles is used in this report. In the first cycle, which focused on adults, female respondents completed a questionnaire in which they were asked whether or not their menstrual periods had begun and if so, at what age. In Cycles II and III, which dealt with 6-through 11-year-olds and 12-through 17-year-olds, respectively, information on menarche used in this report was supplied by the parents.